This invention generally relates to electrical connectors for semiconductor components. More particularly, it relates to a connector between an integrated circuit chip and a substrate. Even more particularly, it relates to a connector that provides a high degree of relief from thermal stress to provide a very reliable joint between an integrated circuit chip or package and a thermal expansion mismatched substrate.
Reliable interconnection of semiconductor integrated circuit chips and supporting substrates depends on avoiding stresses, including thermal expansion stresses, that can crack interconnects. Usually integrated circuits are mounted on supporting substrates made of material with a coefficient of thermal expansion that differs from the coefficient of thermal expansion of the material of the integrated circuit. For example, the integrated circuit may be formed of monocrystalline silicon with a coefficient of thermal expansion of 2.5xc3x9710xe2x88x926 per xc2x0C. and the supporting substrate may be formed of a ceramic material, such as alumina, with a coefficient of thermal expansion of 5.8xc3x9710xe2x88x926 per xc2x0C. In operation, the integrated circuit chip generates heat which raises the temperature of both the chip and the supporting substrate. Because of different temperatures and different coefficients of thermal expansion, the chip and substrate expand and contract different amounts. This difference in expansion imposes stresses on connections, such as the relatively rigid C4 solder bumps that are frequently used to provide an area array interconnection between a chip and a substrate. The stress on the solder bumps is directly proportional to (1) the magnitude of the temperature difference, (2) the distance of an individual bump from the neutral or central point of the solder bump array, and (3) the difference in the coefficients of thermal expansion of the material of the semiconductor device and the substrate, and inversely proportional to the height of the solder bond, that is the spacing between the IC chip and the support substrate.
Several factors are currently compounding the problem. As the solder bumps become smaller in diameter in order to accommodate the need for a greater density of interconnects between chip and substrate, the overall height of each solder bump decreases, reducing the fatigue life of the solder bumps. In addition, integrated circuit chip sizes are increasing which increases the distance of the outer solder bumps from the neutral point of the solder bump array, which in turn reduces the fatigue life of the solder bump. Furthermore, chips are now being directly mounted on substrates, such as PC boards, that have substantially larger coefficients of thermal expansion than ceramic, adding substantially to the stress on connectors. Thus, a better solution is needed that provides a way to reduce thermal stress and to provide a more reliable electrical connection, and this solution is provided by the following invention.
It is therefore an object of the present invention to provide a semiconductor assembly having connections with improved reliability.
It is another object of the present invention to provide a semiconductor assembly having connections that reduce thermal expansion mismatch stress and resist thermal fatigue and cracking due to thermal cycling.
It is another object of the present invention to provide an electrical connection between two substrates wherein a solid conductor in the connection is in substantially movable electrical contact with at least one of the substrates.
It is another object of the present invention to provide a pliable conductive material to facilitate the substantially movable conductor.
It is another object of the present invention to provide a movable conductor that can roll or slide.
It is another object of the present invention to provide a movable conductor that can stretch or bend a magnitude exceeding the elastic limit of a uniform metal.
It is a feature of one embodiment of the present invention that the movable contact is substantially elastic movement.
It is a feature of the present invention that the conductive material is an adhesive that bonds and provides electrical connection while permitting elastic movement of the movable conductor.
It is an advantage of the present invention that thermal stress is avoided despite large disparities in thermal expansion coefficient between chip and substrate.
It is an advantage of the present invention that a semiconductor assembly has good electrical connections while thermal stress is avoided despite large disparities in thermal expansion coefficient between chip and substrate.
These and other objects, features, and advantages of the invention are accomplished by providing a semiconductor assembly comprising a first substrate and a second substrate. The first substrate has a first contact pad and the second substrate has a second contact pad. In addition, the assembly includes a solid conductor and a material bonding the solid conductor wherein the solid conductor is in substantially movable contact with the first contact pad.
In one embodiment an integrated circuit chip has pads electrically connected to pads of a substrate through a metal ball and a pliable material. The pliable material bonds the metal ball in substantially movable contact with the pads. Because the ball is relatively free to move through the pliable material, thermal expansion differences that would ordinarily cause stress in an immovable joint simply cause the ball to roll across the pads as the chip and substrate freely expand or contract at their different rates. Thus, stress is avoided and reliability of the connection is substantially improved as compared with fixed connectors of the prior art.